Advanced Laser Education Content - Part 3 Laser Physics

This page contains treatments advanced education content about laser and treatments. 

Laser Treatment Consumers

A survey by the New American Academy of Dermatology found that 69% of women are bothered by signs of aging;

  • fine lines
  • wrinkles
  • uneven skin tone
  • facial hair
  • Dryness

In the above 45 category, women feel their facial skin makes them look older than they feel inside.

Top Five Cosmetic Procedures:

  • Botox
  • Hyaluronic Acids
  • Laser Hair Removal
  • Chemical Peels
  • Mircrodermabrasion

History of lasers

  • 1900 German physicist Max Planck proposed that “radiant energy may only be absorbed or emitted in discrete packets which he termed quanta or photons
  • 1917 Einstein proposed Quantum Theory of Radiation
  • 1958 Townes and Schawlow applied these principles to Laser
  • 1961 First clinical application of a laser
  • 1966 CO2 laser first used commercially
  • 1983 Theory of Selective Photothermolysis proposed

What is a laser?

  • L - Light
  • A - Amplfication by
  • S - Stimulated
  • E - Emissions of
  • R - Radiation

Laser and Atoms

  • Basics of an atom
  • Absorbing energy
  • Laser/Atom connection
  • Laser light
  • Ruby laser
  • 3 level laser
  • Types of lasers

Inside and atom

The image is a simple atom diagram consisting of a nucleus (containing the protons and neutrons) and an electron cloud. The electrons in the cloud encircle the nucleus in many different orbits.


  • 100 different kinds of atoms in the universe with unlimited combinations
  • How atoms are arranged and bonded together determines the gas, liquid or solid form of any and everything
  • Atoms are in constant motion, vibration and rotation
  • States of excitation are different energy levels of atoms
  • Ground-state energy level when energy is applied, goes to an Excited level
  • The level of excitation depends on the amount of energy that is applied to the atom via heat, light, or electricity

Absorption and emission of energy

Photon is energy released by electron inside atoms in discreet packets


Considering the previous image, more modern views in science of the atom’s construction do not always depict discrete orbits for the electrons, it can be helpful to think of these orbits as the different energy levels of the atom

Example: when applying heat to the atom, we expect that some of the electrons in the lower energy orbits would transition to the higher-energy orbitals farther away from the nucleus.

This is a highly simplified view of it but it actually reflects the core idea of how atoms work in terms of lasers

Once an electron moves to a higher-energy orbit, it eventually wants to return to the ground state. When it does it releases energy as a photon – a particle of light


When the heating element in a toaster turns bright red, the red color is caused by atoms excited by heat, releasing red photons:

laser physics 4.jpg

When you see an older TV screen, what you are seeing is phosphor atoms, excited by high-speed electrons, emitting different colors of light to make up a larger image:

The Laser/Atom connection

A laser is a device that controls the way that energized atoms release photons

Laser – light amplification by stimulated emission of radiation – describes mechanism

All lasers have these essential features:

  • The lasing medium is pumped to get the atoms into an excited state
  • Very intense flashes of light or electrical discharge pump the lasing medium and create a large collection of excited-state atoms (atoms with higher energy electrons)
  • It is necessary to have a large collection of atoms in the excited state for the laser to work efficiently.
  • Atoms are excited to a level that is 2-3 levels above the ground state

This increases the degree of populations inversion:

  • The population inversion is the number of atoms in the excited state vs. the number in ground state

Emissions and wavelengths

Once the lasing medium is pumped, it contains a collection of atoms with some electrons sitting in excited levels

  • The excited electrons have energies greater than the more relaxed electrons

Just as the electron absorbed some amount of energy to reach this excited level, it can also release this energy

The electron at rest can rid itself of some energy, this emitted energy comes in the form of photos (light energy)

  • The photon emitted has a very specific wavelength (color) that depends on the state of the electron’s energy when the photon is released.
  • Two identical atoms with electrons in identical states will release photos with identical wavelengths

Laser Light

Laser light has different properties from normal light, the light release is:

  • Monochromatic – every wavelength is a specific color, the wavelength of light is determined by the amount of energy released when the electron drops to a lower orbit
  • Coherent – each photon moved in same order or step, organized,
  • Directional; columnated and parallel, very tight and strong concentrated beam. Example: a flashlight releases light in many directions, light is very weak and diffused

For all three properties to occur requires:

  • Stimulated emissions – when photon emission is organized

This does not occur in your ordinary flashlight, in a flashlight, all of the atoms release their photons randomly.


The photon that any atom releases has a certain wavelength that is dependent on the energy difference between the excited state and the ground state, stimulated emission can occur. The first photon can stimulate or induce atomic emission such that the subsequent emitted photon (from the second atom) vibrates with the same frequency and direction as the incoming photon.

The other key to a laser is a pair of mirrors, one at each end on the lasing medium. Photons, with a very specific wavelength and phase, reflect off the mirrors to travel back and forth through the lasing medium. In the process, they stimulate other electrons to make the downward energy jump and can cause the emission of more photons of the same wavelength and phase, a cascade effect occurs, and soon we have propagated many photons of the same wavelength and phase.

The mirror at one end of the laser is half-silvered, meaning it reflects some light and lets some light through. The light that makes it through is the laser light.

Ruby Lasers

A ruby laser consists of:

  • flash tube, like that on a camera
  • ruby rod
  • two mirrors, one half-silvered

The ruby rod is the lasing medium and the flash tube pumps it.

Ruby lasers were the first ever made

Ruby Lasers

  • When the flash is off, the laser in in its non-lasing state
  • When the flash tube fires and injects light into the ruby rod, the light excited the atoms in the ruby
  • Some of the excited atoms emit photons
  • Some of these photons run in a directional parallel to the ruby’s axis, they bounce back and forth off the mirrors. As they pass through the crystal, the stimulate emission in other atoms.
  • Monochromatic, single-phase, columnated light leaves the ruby through the half-silvered mirror – laser light

Types of lasers

There are many different types of lasers, the laser medium can be solid, gas, liquid or semiconductor

Lasers are commonly designated by the type of lasing material employed:

  • Solid-state lasers – have lasing materials distributed in a sold matrix (such as the ruby orneodymium yttrium-aluminum garnet “Yag” lasers).
  • The neodymium yttrium-aluminum (Yag) laser emits infrared light at 1,064 nanometers (nm) A nanometer is 1X10-9 meters.
  • Gas lasers – (helium and helium-neon, HeNe, are the most common gas lasers) have a primary output of visible red light.
  • CO2 lasers emit energy in the far-infrared and are used for cutting hard materials.
  • Excimer lasers – (the name is derived from the terms excited and dimmers) use reactive gases such as chlorine and fluorine, missed with inert gases such as argon, krypton or xenon. When electrically stimulated, a pseudo molecule (dimmer) is produced. When lased, the dimer products light in the ultraviolet range.
  • Dye lasers use complex organic dyes such as rhodamine 6G, in liquid solution or suspension as lasing media. They are tunable over a broad range of wavelengths.
  • Semiconductor lasers – sometimes called diode lasers, are not solid-state lasers. These electronic devices are generally very small are use low power. They may be build into larger arrays, such as the writing source in some laser printers or CD players

Spontaneous Emission

  • Electrons exist in a ground (unexcited) state
  • Input energy causes electrons to get excited and jump to a higher orbit (higher energy level)
  • When they get un-excited, they emit a bundle of energy (a photon) and randomly return to their ground state.
  • The emitted photons have different wavelengths, move in different directions and are not in phase
  • This event occurs in nature, like aurora borealis or cooling coals in a fire

Stimulated emission

  • When a photon is released from an exited atom and interacts with another excited atom, triggering the second atom to release two photons.
  • This process can repeat over and over
  • This is known as stimulated emission
  • All the photons are identically in energy, direction and phase
  • This is what happens in a laser


Radiation: The process of energy moving from one location to another in the form of waves or particles

Non-ionizing radiation: when an electron moved from a lower energy orbit to a higher energy orbit

  • Examples: visable, infrared, microwaves, radio waves, lasers

Ionizing radiation: (destructive radiation)when the radiant energy carried enough energy to remove and electron from an atom

  • Examples: gamma waves, x-rays, alpha, beta particles

Electromagnetic spectrum

Electromagnetic spectrum

  • The distance between two consecutive peaks or troughs is called the wavelength
  • Since the wavelength is a measure of distance, it is measure in meters
  • Visible wavelengths are so small that they are measured in billionths of a meter (nanometers, 10 to the negative 9)
  • Frequency is the measure of the number of waves that pass a given point in a specified amount of time
  • When the wavelength increase, the frequency decreases and the energy decreases

How light interacts with skin

  • Reflection
  • Scatter
  • Transmission
  • Absorption - with lasers we are interested in the light that is absorbed into the skin

Selective absorption

  • Short wavelengths are very destructive
  • Shorter wavelengths have higher energy, less depth and higher frequency
  • Longer wave lengths have less energy but go down deeper under the skin, lower frequency

Wavelengths and chromophores

Our body has three chromophores that wavelengths are attracted to:

  • Melanin – brown
  • Hemoglobin – red
  • Water

For a more effective treatment, we need to understand which wavelength is more readily absorbed by which chromophore

The absorption scale is one tool utilized to decide which laser should be used for a particular treatment

Wavelengths and chromophores

Absorption scale

Laser terminology

  • Selective photothermolysis: the process of selective targeting and damaging particular chromophore while leaving the surrounding tissue intact
  • Light is absorbed by the chromophore and is transformed to heat
  • Heat destroys the target without affecting the surrounding tissue
  • Thermal relaxation time: the time required for a target to cool to 50% of the maximum temperature achieved during laser pulsing
  • The target needs to be destroyed before the thermal relaxation time
  • Thermal Destruction time: the time required to thermally damage the target while leaving the surrounding tissue intact
  • Radiation - the process of emitting energy in the forms of waves ir particles
  • Joules – energy emitted or transferred in the form of radiation
  • Watts – power, the rate of energy flow, amount of energy emitted per second
  • Fluence – joules per square centimeter
  • Hertz – speed, the number of pulses per second being delivered

4 basic laser parameters

  • Wavelength – the distance between two consecutive peeks or troughs. Determines the color of the laser beam, measure in nanometers
  • Pulse duration – also known as pulse width: the amount of time it takes to deliver the laser energy measured in milliseconds
  • Spot size – determined by the diameter of the laser beam being emitted, measured in millimeters
  • Fluence – measures the amount of energy delivered per unit area, measured in joules per centimeter squared

4 components of a laser

  • Lasing medium – solid, liquid, gas, diode
  • Optical cavity
  • Power source – electricity, flash lamp, chemicals or another laser
  • Delivery system – articulated arms, optical fibers, lenses